1. Field of the Invention
The invention relates to an actuation device for a friction clutch arranged in the drive train of a motor vehicle including an engagement/disengagement arrangement having a pressure fluid cylinder and a pressure fluid piston axially displaceable therein, the engagement/disengagement arrangement being effectively connected to a pressure fluid source via a control valve actuatable as a function of a first control parameter representing a required axial position and a second control parameter representing the actual axial position of the engagement/disengagement arrangement.
2. Description of the Related Art
A known actuation device is known, for example, from German reference DE 33 21 578 C2 which discloses a positioning arrangement comprising a vacuum servo-force amplifier. The vacuum servo-force amplifier is constructed as a vacuum braking force amplifier arranged outside of a housing bell and is integrally configured with a pneumatic power cylinder and a control valve. A piston is axially guidably arranged with an elastic diaphragm in the pneumatic power cylinder and separates the pneumatic power cylinder into two working chambers. A first working chamber is configured as vacuum chamber and connected to an induction line system of an internal combustion engine. The second working chamber is used as control chamber and is selectively connectable by the control valve either to the vacuum chamber, i.e., the first working chamber, or to the atmosphere via a pressure-balance opening. The vacuum servo-force amplifier is driven by a control rod which is axially displaceable by a motor-driven cam. The position of the control valve is switched by the axial displacement of the control rod. The piston follows the motion of the control rod with an amplified force. The motion of the piston acts directly on a hydraulic master cylinder which in turn acts on a slave cylinder which is arranged outside the housing bell and acts, in turn, on an engagement/disengagement fork associated with the engagement/disengagement bearing arrangement. The control valve has a valve body which interacts with a flexible valve seat. A connecting duct is provided in the valve seat and the connection between the control chamber and the ambient air takes place via this connecting duct, provided the valve body is not pressed against the elastic valve seat in order to close the connecting duct by means of the valve body. A further connecting duct connects the control chamber with the vacuum chamber.
In addition, another known actuation device has a pneumatic power cylinder comprising a pressure fluid power cylinder arrangement. This known actuation device is fastened on the outside of a housing bell as an integral unit which comprises a pneumatic power cylinder, a hydraulic slave cylinder and the control valve. The piston of the pneumatic power cylinder is installed on a rod element forming the piston of the hydraulic slave cylinder. The rod element is connected to a push-rod which extends inside the housing bell and acts on an engagement/disengagement fork associated with the engagement/disengagement bearing arrangement. A master cylinder which is actuatable by a clutch pedal and a control input for the control valve are connected to the slave hydraulic cylinder. The control valve controls the supply of compressed air to the pneumatic power cylinder and the release of air from the pneumatic power cylinder as a function of the hydraulic pressure present at the control input so that a specified hydraulic pressure, determined by a compression spring arrangement, is set at the control input. In this arrangement, the slave hydraulic cylinder is used as a measurement cylinder which records the position of the rod element. Since the rod element is also connected to the push-rod which extends inside the housing bell and acts on an engagement/disengagement fork, the slave hydraulic cylinder indirectly records the position of the engagement/disengagement bearing arrangement. When the master cylinder is actuated, forces are exerted directly on the rod element, and therefore on the engagement/disengagement bearing arrangement, via the hydraulic slave cylinder used as the measurement cylinder. These forces are additional to the actuation forces of the pneumatic power cylinder due to the supply of compressed air to the latter.
It has been deemed advantageous to arrange the pressure fluid power cylinder of these known actuation devices within the housing bell. However, it is to be expected that the compact construction and known hydraulic control of these actuation devices will involve relatively high manufacturing costs with respect to the sealing requirements. Because, on the other hand, the engagement/disengagement function is to be achieved by means of an intrinsic engagement/disengagement arrangement so that the control line will not have to transmit any high forces, an economic and reliable alternative is desirable.